1. Field of the Invention
The present invention relates to a planographic printing plate precursor, and more particularly to a positive planographic printing plate precursor having a recording layer that is rendered alkali-soluble by exposure to an infrared laser.
2. Description of the Related Art
The development of lasers in recent years has been remarkable. In particular, high-output, compact solid-state lasers and semiconductor lasers having an emission range in the near infrared to infrared range are being developed. These lasers are extremely useful as exposure light sources when a printing plate is formed directly on the basis of digital data from a computer or the like.
Positive planographic printing plate precursors for exposure to IR lasers include as essential components a binder resin that is soluble in an aqueous alkali solution, and an infrared (IR) dye that absorbs light to generate heat. In the non-exposed area (image area) of the precursor, the IR dye acts as a dissolution inhibitor that interacts with the binder resin to substantially lower the solubility of the binder resin. In the exposed area (non-image area) of the precursor, however, the interaction of the IR dye with the binder resin is weakened by the heat generated and the binder resin dissolves in alkali developer, whereby the precursor is processed into a planographic printing plate.
However, the difference in various processing conditions between resistance to dissolution of the non-exposed area (image area) in a developer and solubility of the exposed area (non-image area) is still insufficient, and there are problems in that varying processing conditions tends to produce overdevelopment or insufficient development. In addition, it is easy for the surface of the precursor to be compromised by, for example, fine scratches formed in the surface of the precursor when the precursor is handled. When such fine scratches are formed in the surface of the precursor and the surface of the precursor is slightly compromised, the surface solubility of the precursor increases, and when developed, the non-exposed area (image area) of the precursor dissolves to form scratch marks. This lowers printing durability and interferes with ink adhesion to the printing plate.
These problems stem from a fundamental difference in the plate-making mechanism between positive planographic printing plate precursors for IR lasers and positive planographic printing plate precursors, which are processed into printing plates by being exposed to UV light. Specifically, positive planographic printing plate precursors processed into printing plates by being exposed to UV light include as essential components a binder resin soluble in an aqueous alkali solution and an onium salt or quinonediazide compound. The onium salt or the quinonediazide compound not only interacts with the binder resin to lower the solubility of the binder resin at the non-exposed area (image area), but also promotes dissolution by being decomposed by light to generate an acid at the exposed area (non-image area). The onium salt or the quinonediazide compound thus plays two roles.
In contrast, in positive planographic printing plate precursors for IR lasers, IR dyes act only as dissolution inhibitors at the non-exposed area (image area), and do not promote the dissolution of the binder resin at the exposed area (non-image area). In order to produce a difference in solubility between the non-exposed area and the exposed area, a binder resin having a high solubility in alkali developer must be used in the precursor, and this leads to the problems of poor scratch resistance and unstable conditions before development of the precursor.
As means to overcome these problems, Japanese Patent Application Laid-open Publication (JP-A) No. 10-250255 proposes multiple thermosensitive layers. However, these layers do not substantially utilize changes in solubility in the key process of image formation, but utilize a function in which ablation of an upper layer alters the penetrability of the developer into the layers. Therefore, various problems arise, such as a drop in laser output due to contamination of the optical system resulting from ablation, and contamination of the interior of the device.
JP-A No. 2000-35662 discloses multiple thermosensitive layers including an upper layer and a lower layer, with each of the upper and lower layers containing a light-heat converting material. When carbon black is used as the light-heat converting material as in the embodiments described in the application, problems due to ablation easily arise when sensitivity is raised by increasing the amount of carbon black in the uppermost layer. On the other hand, when the amount of carbon black in the uppermost layer is lowered to prevent ablation, there are problems in that sensitivity is significantly reduced. Thus, it has been difficult to overcome the preceding problems simply by adding a light-heat converting agent to the two layers.